Thermochemical analysis of core correlation and scalar relativistic effects on molecular atomization energies

Core correlation and scalar relativistic contributions to the atomization e nergy of 120 first- and second-row molecules have been determined using cou pled cluster and averaged coupled-pair functional methods and the MTsmall c ore correlation basis set. These results are used to parametrize an improve d version of a previously proposed bond order scheme for estimating contrib utions to atomization energies. The resulting model, which requires negligi ble computational effort, reproduces the computed core correlation contribu tions with 88%-94% average accuracy (depending on the type of molecule), an d the scalar relativistic contribution with 82%-89% accuracy. This permits high-accuracy thermochemical calculations at greatly reduced computational cost. (C) 2000 American Institute of Physics. [S0021-9606(00)30328-2].